1. Field of the Invention
The present invention relates to an ADSL (Asymmetric Digital Subscriber Line) modem capable of realizing stable high-speed data communication without signal superposition between ADSL modems or impedance mismatching even in the case where a plurality of ADSL modems are connected to a single telephone line. An ADSL modem may alternatively be referred to as an ADSL transceiver.
2. Description of the Related Art
A conventional ADSL modem is used in a system configuration for point-to-point connection. In SOHO (Small office and Home Office) applications, for example, the DSLAM (Digital Subscriber Line Access Multiplexer) in a switching office and an ADSL router in a business office or a user are connected point-to-point to each other although a multipoint connection, using a LAN, is used within the office or the user""s home.
The current trend toward the ADSL modem being built in personal computers, however, tends to lead to a situation in which the ADSL modem is combined or built in with a standard analog modem built in the personal computer. In other words, the analog modem and the ADSL modem are often formed on a single board or within a single module. As a result, a personal computer having an analog modem and an ADSL modem built therein is connected to a single telephone line cable.
Further, the extension of the internet has given rise to many cases in which the user connects a plurality of personal computers (including a built-in ADSL modem and a built-in analog modem) to a single telephone line in his home or office.
This situation has posed the problem that, in the case where a given ADSL modem starts data communication while another ADSL modem is in data communication, signals are superposed on the telephone line, with the result that the ADSL modems cannot perform the data communication.
On the other hand, a NCU (Network Control Unit) is built in the conventional analog modem for the switching circuit which requires the call control, and therefore the electrical insulation can be maintained in an on-hook state even when the telephone line is physically in a multi-point connection. The ADSL for which the call control is not required, however, has no NCU. In the case where a plurality of conventional ADSL modems are connected to a single telephone line, therefore, the impedance of 100xcexa9 cannot be maintained thereby making high-speed communication impossible.
In view of this problem, the object of the present invention is to provide an ADSL modem capable of realizing a stable high-speed data communication without signal superposition between ADSL modems or impedance mismatching even in the case where a plurality of ADSL modems are connected to a single telephone line in a SOHO application or in an ordinary user""s home.
In order to achieve the object described above, according to a first aspect of the invention, there is provided an ADSL modem, connected to the telephone line, comprising a detector for detecting whether another ADSL modem connected to the telephone line is using the ADSL frequency band and a control unit for preventing the application of signal energy to the ADSL frequency band upon detection that another ADSL modem is using the ADSL frequency band.
As long as another ADSL modem is using the ADSL band, no signal energy is applied to the ADSL frequency band. Therefore, the ADSL signals are not superposed along the telephone line and a terminal impedance of 100xcexa9 can be maintained.
According to a second aspect of the invention, there is provided an ADSL modem wherein the detector includes a FFT demodulation unit for performing FFT demodulation of one carrier in the input signal from the telephone line, a normalization circuit for generating a normalized demodulation signal by normalizing the output signal of the FFT demodulation unit, a symbol delay buffer for generating a one-symbol delay signal delayed by one symbol behind the output signal of the normalization circuit, a complex conjugate unit for generating a complex conjugate of the output signal of the normalization circuit, a multiplier unit for multiplying the one-symbol delay signal with the complex conjugate and generating a difference signal, and a decision circuit for deciding that the ADSL band of the telephone line is occupied by another ADSL modem in the case where the difference signal remains not more than a predetermined threshold value for a period corresponding to a specific number of symbols.
In the case where the difference signal remains not more than a predetermined threshold for a predetermined number of symbols, it is decided that the particular signal is free of variations in both phase and frequency, and therefore it is decided that an ADSL signal and not noise has been received.
According to a third aspect of the invention, there is provided an ADSL modem in which the aforementioned one carrier in the input signal is preferably the 64th carrier corresponding to a predetermined tone in the ADSL frequency band.
The 64th carrier corresponds to the signal constantly continuing to produce the tone, and therefore by utilizing this carrier, it can be positively decided whether the ADSL band is occupied or not.
According to a fourth aspect of the invention, there is provided an ADSL modem in which the decision unit described above can preferably decide that the ADSL band of the telephone line is occupied by another ADSL modem upon detection of a prescribed number of periods, each period being defined as the state in which the phase component of the difference signal remains not more than a predetermined threshold value continuously for at least a specified number of symbols but not for a subsequent symbol.
As a result, it can be decided more positively that the ADSL band of the telephone line is occupied, for communication, by another ADSL modem.
According to a fifth aspect of the invention, there is provided an ADSL modem in which the specified number of symbols is preferably three.
The ITU-T G.992.2 Annex-C standard stipulates that the period free of the effect of the near-end crosstalk is three symbols, minimum, per period of noise, and therefore by utilizing this fact, it can be decided whether the ADSL band is occupied or not.
According to a sixth aspect of the invention, there is provided an ADSL modem wherein the control unit includes a line transformer connected to the telephone line and a terminal resistance switching circuit for switching the resistance value of the terminal resistor connected between the secondary of the line transformer and an ADSL signal transmission unit, the control unit further preferably including an impedance control unit by which a high impedance is maintained between the signal transmission unit and the secondary of the line transformer by controlling the terminal resistance switching circuit when the detector detects that the telephone line is occupied by another ADSL modem and a low impedance is maintained between the signal transmission unit and the secondary of the line transformer by controlling the terminal resistance switching circuit when the detector detects that the telephone line is not occupied by another ADSL modem.
In the case where the telephone line is occupied by another ADSL modem, a high impedance is maintained between the signal transmission unit and the secondary of the line transformer by the terminal resistance switching circuit, and therefore the superposition of the ADSL signals on the telephone line is positively prevented while at the same time positively keeping the terminal resistance at the impedance of 100xcexa9.
According to a seventh aspect of the invention, there is provided an ADSL modem wherein the detector decides whether another ADSL modem occupies the ADSL frequency band by an exchange of messages by polling other ADSL modems outside the ADSL band instead of detecting the signal in the ADSL band.
By message exchange, it can be decided, by direct inquiry, whether the ADSL band is occupied by another ADSL modem or not.
According to an eighth aspect of the invention, there is provided an ADSL modem wherein the message exchange by polling is preferably performed in the operating frequency band of an analog modem.
In the case where the ADSL modem has the function of an analog modem at the same time, the message exchange can be performed in the operating band of the analog modem.
According to a ninth aspect of the invention, there is provided an ADSL modem of the eighth aspect wherein the detector includes an analog modem, which in turn includes a 600xcexa9 terminal circuit, a DC bypass circuit connected in parallel to the 600xcexa9 terminal circuit, and a switch for turning off the DC bypass circuit in on-hook state and turning on the DC bypass circuit in off-hook state of the analog modem, the message exchange being preferably performed by polling using the audio band when the analog modem is in on-hook state.
In view of the fact that the message exchange is performed by polling using the audio band in the on-hook state of an analog modem, the call control is not carried out for the switching office even though messages are exchanged.
According to a tenth aspect of the invention, there is provided an ADSL modem wherein the message exchange by polling is carried out in the home phone line networking alliance band of an analog modem.
In the case where the ADSL modem has a communication function in the HomePNA band, the message exchange by polling is possible using the HomePNA band.
According to an 11th aspect of the invention, there is provided an ADSL modem wherein in the case where the ADSL band is occupied by another ADSL modem, the communication with other ADSL modems is performed in the home phone line networking alliance band, and the message received through the home phone line networking alliance band is multiplexed and sent out to the ADSL band.
Consequently, even in the case where the ADSL band is occupied by another ADSL modem, the communication is possible using the ADSL band without waiting until the end of the occupancy.